/*
 * @lc app=leetcode.cn id=617 lang=cpp
 *
 * [617] 合并二叉树
 */

// @lc code=start
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
#include <bits/stdc++.h>
using namespace std;
class Solution
{
    TreeNode *dfs(TreeNode *root1, TreeNode *root2)
    {
        if (!root1 && !root2)
            return nullptr;
        if (root1 && root2)
        {
            root1->val += root2->val;
            root1->left = dfs(root1->left, root2->left);
            root1->right = dfs(root1->right, root2->right);
            return root1;
        }
        TreeNode *t1 = root1 ? root1 : root2;
        TreeNode *t2 = root1 ? root2 : root1;
        // 保证 t1 不为 nullptr, t2 是 nullptr
        // dfs(t1->left, nullptr);
        // dfs(t1->right, nullptr);
        return t1;
    }

public:
    TreeNode *mergeTrees(TreeNode *root1, TreeNode *root2)
    {
        return dfs(root1, root2);
    }
};
// @lc code=end
